Damping remains a hot-topic in design of offshore wind turbines as turbine and monopile sizes grow. For these new designs the contribution of fatigue loads in parked conditions have increased. In addition the larger impact of wave loads on XL monopiles has given wind wave misalignment a bigger role in fatigue in the cross-wind direction. Both fatigue mechanisms are linked by the absence of aerodynamic damping, with damping coming purely from the soil, the water and the structural steel.

For large monopiles any additional damping in the soil will thus have a beneficial effect on the fatigue life and ultimately the cost of the turbine. However, in contrast to its importance, the damping-contribution of the soil is only based on a best estimate and a limited amount of measurements. To bridge this knowledge gap OWI-lab has invested in monitoring campaigns on multiple operational wind turbines to determine damping. As a result OWI-lab currently has a database of thousands of damping estimates for different sites in the North-Sea.

In this contribution these results will be used to discuss the role of damping for offshore wind turbines. In particular we focus on the contribution of the soil to total damping. The variation of damping due to different soil conditions is made visible by looking at sideways (cross-wind) damping from different sites in the North-Sea.

Method

OWI-lab operates 5 monitoring systems spread across all Belgian offshore wind farms. These monitoring systems use accelerometers and state-of-the-art signal processing tools to continuously determine the damping of the wind turbine in both the Fore-Aft and sideways directions. Combined OWI-lab has collected close to 500.000 damping values over a period of 5 years.

In general the contribution of aerodynamical damping is dominant over the contribution of other sources to damping. However in parked conditions, and for sideways damping, the contribution of the aerodynamic damping is significantly lower. This means that sideways damping gives us a direct measure of the damping coming from the soil and the hydrodynamics.

Results

In this contribution the results of monitoring over a period of 5 years will be condensed to focus on the role of soil damping. The long-term results of measured sideways damping are used to assess the soil damping at different sites. The results from this assessment will then be compared between different wind turbine sites within a farm. The results show a spread on the amount of soil damping for different sites, even with similar monopile designs, giving motivation to use a site-specific damping coefficient during design.

Conclusions

Damping of offshore wind turbines is generally dominated by the contribution of aerodynamic damping. However, with growing monopiles and larger turbines the role of fatigue during low-damping conditions has grown. As a result the importance of the soil damping is increasing in the design of offshore wind turbines. This contribution quantified the soil damping at different offshore wind turbine sites in the Belgian North Sea. The variability of soil damping over these different sites motivates the use of a site-specific soil damping coefficient in order to further optimize design of future wind turbines.

Objectives

In this contribution a strategy to assess damping of offshore wind turbines using a monitoring system will be shown. Results will illustrate the variability of the aerodynamic damping, and highlight low-damping conditions where aerodynamic damping only plays a minor role. From these results it will focus on the role of soil damping in the total damping of offshore wind turbines and the role of damping during design. A comparison of the soil damping at different sites of the same farm, will motivate the use of site-specific damping during design.